docs: add a scriv-managed changelog

This will be useful for tracking changes across releases.

This partly addresses overhangio/tutor#746

CHANGELOG.md

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+i# Changelog
+
+This file includes a history of past releases. Changes that were not yet added to a release are in the [changelog.d/](./changelog.d) folder.
+
+<!--
+⚠️ DO NOT ADD YOUR CHANGES TO THIS FILE! (unless you want to modify existing changelog entries in this file)
+Changelog entries are managed by scriv. After you have made some changes to this plugin, create a changelog entry with:
+
+    scriv create
+
+Edit and commit the newly-created file in changelog.d.
+
+If you need to create a new release, create a separate commit just for that. It is important to respect these
+instructions, because git commits are used to generate release notes:
+  - Modify the version number in `__about__.py`.
+  - Collect changelog entries with `scriv collect`
+  - The title of the commit should be the same as the new version: "vX.Y.Z".
+-->
+
+<!-- scriv-insert-here -->
+

changelog.d/20230519_161836_regis.md

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+- [Improvement] Add a scriv-compliant changelog. (by @regisb)

changelog.d/scriv.ini

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+[scriv]
+version = literal: tutornotes/__about__.py: __version__
+categories =
+format = md
+md_header_level = 2
+new_fragment_template =
+    <!--
+    Create a changelog entry for every new user-facing change. Please respect the following instructions:
+    - Indicate breaking changes by prepending an explosion 💥 character.
+    - Prefix your changes with either [Bugfix], [Improvement], [Feature], [Security], [Deprecation].
+    - You may optionally append "(by @<author>)" at the end of the line, where "<author>" is either one (just one)
+    of your GitHub username, real name or affiliated organization. These affiliations will be displayed in
+    the release notes for every release.
+    -->
+
+    <!-- - 💥[Feature] Foobarize the blorginator. This breaks plugins by renaming the `FOO_DO` filter to `BAR_DO`. (by @regisb) -->
+    <!-- - [Improvement] This is a non-breaking change. Life is good. (by @billgates) -->
+entry_title_template = {%% if version %%}v{{ version }} {%% endif %%}({{ date.strftime('%%Y-%%m-%%d') }})